1. Field of the Invention
The invention relates to a unit comprising an electric drive motor and a radial piston pump.
2. Description of the Prior Art
U.S. Pat. No. 5,105,685 a unit of this kind is disclosed which has an electric drive motor and a radial piston pump, and in which a shaft, carrying an armature together with a commutator of the drive motor and merging at one free end into an eccentric for driving at least one pump piston, is mounted by means of two ball bearings, which are arranged one on each side of the combination comprising the commutator and the armature. The commutator in this arrangement is situated closer to the eccentric than the armature. A brush-holder for the commutator is fastened to a component receiving the ball bearing situated between the commutator and the eccentric. A disadvantage in this embodiment is the relatively great distance between this ball bearing and the eccentric projecting freely from the shaft, because this relatively great distance gives rise to a heavy bending load both on the eccentric and on the shaft. As-a reaction to this high bending load, the shaft is also subjected to a bending load in the region of the commutator, with consequential elastic deformation of the shaft and correspondingly disadvantageous radial displacements of that part of the armature which is adjacent to the commutator. The occurrence of flexural vibrations on the shaft for this reason and of troublesome noise because of unavoidable play in the two ball bearings cannot be precluded.
U.S. Pat. No. 4,568,131 another unit is disclosed which comprises an electric drive motor and a radial piston pump and has two ball bearings, wherein the commutator, the armature and the eccentric are arranged in that order between the two ball bearings. In this arrangement, a shaft stub extends from the eccentric and penetrates a short distance into one of the ball bearings. Another shaft stub adjoining the commutator penetrates into the other ball bearing. Although the one ball bearing is arranged in an advantageous manner in the immediate vicinity of the eccentric, nevertheless the distance between the two ball bearings is on the other hand relatively great and the mass of the armature is situated substantially centrally between the ball bearings, so that the disadvantage exists that considerable deflection of the shaft in the region of the armature cannot be precluded. The shaft therefore has low flexural rigidity, so that pumping operation may occur at the so-called supercritical speed with the disadvantage that on transition from an uncritical speed to the supercritical speed considerable radial displacements of the armature from a design axis assigned to it cannot be precluded. This can also lead to troublesome noise generation in the ball bearings because of unavoidable play.
In European Patent No. 0 509 883 B1 a unit is disclosed which comprises a drive motor and a radial piston pump and has a stator, an armature, a shaft carrying the armature and an eccentric, and three ball bearings for the shaft. The first ball bearing is here situated between the armature and the eccentric, the second bearing next to the eccentric in the region of one end of the shaft, and the third ball bearing at the other end of the shaft in the region of a commutator associated with the armature. The arrangement of these three ball bearings is advantageous because elastic bending of the shaft is now limited. In the region of the armature the shaft therefore has a harder spring constant and accordingly a higher natural oscillation frequency, so that the unit is preferentially operated in the subcritical range. A disadvantage is the cost of the third ball bearing.